1. Field of the Invention
The present invention relates to an optical communications system and, more specifically, to an optical communications system in which an exchange and a subscriber are linked via an optical fiber cable, a single wavelength of 1.3 .mu.m being used for bi-directional communications and a wideband signal being transmitted to the subscriber.
2. Description of the Related Art
A known system for using an optical fiber cable to transmit a wideband signal to a subscriber is the WDM (wavelength division multiplexing) system, in which wavelength multiplexing is done of a 1.55-.mu.m video frequency division multiplexed (FDM) signal and a 1.3-.mu.m low-speed data bi-directional burst-multiplexed signal.
In the WDM system of the past, a multichannel video frequency division multiplexed (FDM) signal is converted at the exchange to a 1.55-.mu.m optical signal using an electrical-optical converter, this optical signal passing through an optical mixer/divider and then being output to the optical transmission path. At the subscriber side, the 1.55-.mu.m optical signal from the optical transmission path is separated using an optical mixer/divider, after which it is restored to a video signal by means of an optical-electrical converter.
Low-speed signals, such as voice or low-speed data, are used for bi-directional communications between the exchange and the subscriber; in these communications, systems such as TCM (time compression multiplexing), which performs time switching between transmission and reception, are used. Specifically, during the period of transmission from the exchange, the low-speed signal is converted to a 1.3-.mu.m optical signal using an electrical-optical converter, this signal passing through an optical coupler and then through an optical mixer/divider, at which it is wavelength multiplexed with the above-noted 1.55-.mu.m video optical signal (thereby becoming a 1.3 .mu.m+1.55 .mu.m signal), this signal then being output to the optical fiber transmission path. At the subscriber side, this multiplexed signal is separated into the 1.3-.mu.m and the 1.55-.mu.m optical signals using an optical mixer/divider, the above-noted 1.3-.mu.m optical signal being restored to a low-speed signal by means of an optical-electrical converter.
On the reverse side, during the receiving period of the exchange, that is, during the period in which the subscriber is transmitting, the low-speed signal at the subscriber side is converted to a 1.3-.mu.m optical signal, using an electrical-optical converter, this optical signal passing through an optical coupler and being fed to an optical mixer/divider, the multiplexed signal output of which (1.3 .mu.m+1.55 .mu.m) being output to a single optical fiber transmission path. At the exchange, the above-noted multiplexed signal is separated into a 1.3-.mu.m optical signal and a 1.55-.mu.m optical signal using an optical mixer/divider, the 1.3-.mu.m optical signal being fed via an optical coupler to an optical-electrical converter, which restores it to the original low-speed signal.
In this manner, with regard to the low-speed signal, bi-directional communications are performed by alternately switching between transmission and reception between the exchange and the subscriber. By using optical mixer/dividers, the low-speed signal is either mixed with or separated from the high-speed signal, such as the above-noted video signal, thereby to enable signal transmission without mutual interference between the two signals.
However, in the above-described system, in order to perform wavelength division multiplexing of the video signal and a low-speed signal, it is necessary to have two types of optical components, those for 1.55 .mu.m and those for 1.3 .mu.m, and it is further necessary to have optical mixer/dividers for the purpose of mixing and separating these signals. In addition, it is necessary to have a circuit device for the purpose of time division multiplexing of transmission, and of reception, of the low-speed signal, these hardware requirements making this past system costly and complex.
In addition, with regard to the subscriber signal, while there is a desire to have wideband signals such as video signals sent from the exchange to the subscriber, in the reverse direction, the signal from the subscriber to the exchange is a low-speed, narrowband signal, such as request signals and telephone signals; thus it is necessary to consider the extremely asymmetrical nature, in terms of the signal transmission directions, of such prior art systems.